This invention relates to a spectroscopic apparatus and method. More particularly, the invention relates to a glow discharge lamp and to an apparatus and a method for spectroscopically analysing substances by means of fluorescence detection.
Glow discharge lamps, having a hollow, tubular anode and a planar, transverse cathode, are known to the applicant. These known lamps generate spectral radiation by creating an excited atomic cloud by cathodic sputtering, and are utilised as primary sources of radiation. It is a feature of these lamps that, as they are intended to be only primary radiation sources, they have only one window such that generated radiation is emitted from the lamp in only one direction.
Such known glow discharge lamps may be used to analyse an unknown substance by absorption or fluorescence spectroscopy techniques. With these techniques, the unknown substance is stimulated so that it emits primary radiation characteristic of its component elements, and this primary radiation is passed through an atomic cloud of a particular reference element, the presence and concentration of which in the unknown substance is to be determined. The atoms of the atomic cloud then selectively absorb components of the primary radiation and are caused to fluoresce emitting secondary radiation characteristic of the atoms of the atomic cloud. The absorption or the fluorescence is then detected in order to determine whether or not the reference element is included in the unknown substance. With fluorescence spectroscopy, the excited atoms of the atomic cloud may re-emit radiation of the same wavelength as that absorbed by the atoms, such re-radiation being termed "resonance radiation". Alternatively radiation of a larger wavelength may be re-radiated. An apparatus and a method for detecting resonance radiation is described in British Pat. No. 1,042,129 granted to the Commonwealth Scientific and Industrial Research Organisation.
It is thus often required that an atomic cloud be generated such that radiation from the external source may pass through the cloud, e.g. in absorption spectroscopy; or, e.g. with fluorescence spectroscopy, that the atomic cloud be illuminable by primary radiation from the external source, and that secondary radiation of the cloud be emitted along a path that is transverse to that of the primary radiation.
Instead of the atomic cloud being generated by cathodic sputtering it may be generated by thermal means, although cathodic sputtering is the preferred manner. However, when the cloud is generated by cathodic sputtering the atoms are sufficiently excited to emit inherent radiation, which is of a broad bandwidth that detracts from the accuracy and sensitivity of detection.
It has been found that with an atomic cloud generated by cathodic sputtering in a suitable lamp the intensity of inherent radiation decays much more rapidly than the concentration of neutral atoms. The concentration of neutral atoms has been found to obey the equation EQU N(t) = Noe.sup.-k(P)t
where N(t) is the number density of atoms at time t, No is the number density of atoms at time t=o, and k(P) is a rate constant that is a function of the pressure P extant in the lamp.
The rate constant has been found to be inversely related to the pressure P whereas the decay time of radiation emitted by the atomic cloud is substantially independent of the pressure. Thus, conditions may be obtained in which the atomic cloud still has a substantial concentration even though its inherent radiation is substantially zero, by suitably pressurizing the lamp in which the atomic cloud is generated.